This invention relates to a system and methods for monitoring of boundaries. More specifically, but without limitation, this invention relates to a security system that transmits vibrations along a waveguide and then senses the vibrations to detect, localize, and/or classify the vibration.
The prior art discloses a number of different means to detect intrusions or other disturbances in a fence or other boundary. One common method is to use taut wire systems. One example of a taut wire system is disclosed in U.S. Pat. No. 4,829,287 to Kerr et al. In such a taut wire system, sensors such as pressure sensors or strain gauges are used to sense changes in the tension of the wire. In this and other systems, because tension is being sensed, a number of sensors are required along the fence to ensure that an intrusion does not go undetected. If there is too great of distance between sensors, then added tension due to an intrusion may go unnoticed. A specially-designed fence for use in a taut wire system may have integrated strain gauges to detect stress changes and vibrations from climbing. This type of system is very expensive to build and especially to maintain. Also, wind, rain, and thermal expansion and contraction cause false alarms. Thus there are numerous potential problems with this approach.
Another example of a prior art approach is to use direct vibration sensors such as geophones. A geophone is attached to the chain-link fence fabric every 20 to 30 feet and wired together in parallel. Using direct vibration sensors such as geophones is very expensive, sensitive to sensor failure, easily vandalized, cannot localize, and has false alarms from environmental noise.
Yet another type of system uses active microwave waveguides. This type of system uses a leaky coaxial cable and an active microwave pulse transmitter to monitor the reflection response along a segment of fence where the cable is woven into a “zig-zag” pattern in the fence fabric. Any change in the fence stress or vibration changes the microwave echo pattern, thus allowing a detection and localization. The shortcomings of this approach are the exposure to vandalism, expense, and maintenance. One example of a leaky coaxial cable system is disclosed in U.S. Pat. No. 4,879,544 to Maki et al. In such a system, two cables are run parallel to one another, one acting as a transmitter, the other acting as a receiver. When the radio frequency signal leaks from the transmitter cable to the receiver cable, a field is created between the two cables. The changes in the field are monitored to determine if an intrusion has occurred. If the cable is cut, then this type of system fails to work and requires repair.
Another type of system uses fiber optic cables. Fiber optic intrusion detection (FOIDS) uses a laser and optical fiber where the interference pattern of the fiber reflections and the laser produce a sensor with very high sensitivity to both stress and vibration. The fiber is woven into the fence fabric and is easily vandalized as well as exposed to environmental degradation. FOIDS suffers from significant false alarms due to its sensitivity and cannot localize within a segment of fiber.
U.S. Pat. No. 6,731,210 to Swanson et al, herein incorporated by reference in its entirety, discloses a fence security system that uses a waveguide made of a simple wire with sensors that process the acoustics wave to localize the intrusion. Swanson et al teaches localization based on time of arrival as well as amplitude. Thus, although Swanson et al may be advantageous over other approaches, problems remain. In particular, what is needed is a simple way to calibrate and localize intrusions, improved immunity to wind and rain noise, improved rejection of nuisance alarms, and reduced localization error.